1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for towing seismic equipment under water.
2. Discussion of the Background
During the past years, the interest in developing new oil and gas production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel. Offshore drilling is an expensive process. Thus, those engaged in such a costly undertaking invest substantially in geophysical surveys in order to more accurately decide where to drill in order to avoid a dry well.
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for the oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 drags an array of seismic detectors provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to a surface 14 of the ocean. The vessel 10 also tows a seismic source assembly 16 that is configured to generate an acoustic wave 18. The acoustic wave 18 propagates downwards toward the seafloor 20 and penetrates the seafloor until eventually a reflecting structure 22 (reflector) reflects the acoustic wave. The reflected acoustic wave 24 propagates upwardly until the same is detected by detector 26.
The streamers 12 are shown in FIG. 2 spreading over a predetermined area. This is called the seismic spread. In order to maintain the plural streamers 12 substantially parallel and at equal distance from each other, various front-end gears are used. Streamers 12 are spread out to a desired width to provide measurements of the geological conditions over an acquisition area.
An example of a front-end gear 30 is shown in FIG. 2. The front-end gear 30 is provided between the vessel 10 and the various streamers 12 and this gear is configured to achieve the desired positioning for the streamer heads. FIG. 2 shows the front-end gear 30 to include cables 32 connected between the vessel 10 and deflectors 34. Deflector 34 is a structure capable of generating the necessary lift when towed to keep the streamers deployed in the transverse direction with respect to the sailing line of the towing vessel 10. Spacers 36 are attached to the cables 32 for distributing the lift force among them in order to obtain a substantially linear profile for the position of the streamer heads.
As said above, to spread the streamers transversely in relation to the seismic vessel, the deflectors 34 are usually used. Such deflectors are traditionally passive devices including one or more wings providing a lift in the required direction. Because of the towing resistance in the water, caused both by the deflector and the towed cables, there are, however, limits to the lift which may be obtained using passive deflectors. Thus, due to the limited lift that can be generated by the deflectors, a width of the seismic spread is also limited, which is undesirable. When the deflector is used in seismic surveys it will in addition be loaded with the streamers to be pulled sideways. Thus, there is a limit to the width of the cable tow with passive deflectors.
Another conventional configuration that is presently in use is shown in FIG. 3. FIG. 3 shows a vessel 40 towing two ropes 42 provided at respective ends with deflectors 44. Plural lead-in cables 46 are connected to streamers 50 (e.g., they may form a single cable). The plural lead-in cables 46 also connect to the vessel 40. The streamers 50 are maintained at desired separations from each other by separation ropes 48. Plural sources 52 are also connected to the vessel 40. However, this configuration introduces a large drag. Although the positioning of the streamers may be good, this configuration introduces high internal forces and high resulting stresses in the lines (cables/ropes) and connection means. This results from the raying pattern and the highly constrained geometry where everything is connected by tensioned lines.
Further, the processing techniques for seismic data require longer and more streamers, which only increase the load (due to their drag) of the surveying vessel that tows the streamers. However, as the towing vessel has a limited power availability, the large drag needs to be reduced. Accordingly, it would be desirable to provide systems and methods that provide the operator of the vessel with the capability to use long streamers and an increased number of streamers if so desired.
As the use of the seismic vessel is expensive, it is advantageous to make the width of the spread as large as possible, with a large number of streamers, so that one vessel pass covers an area as large as possible. In this regard, ultra-wide-tow seismic spread using more than 20 streamers is targeted but is something unfeasible using conventional front-end gear architectures. An illustrative configuration in this case could be one with 30 streamers, each having a length of up to 8000 m and a separation between the streamers in the order of 100 m. A short hand for such configuration is 30×8000×100.
On the other hand, for doing dense acquisition, smaller separation between streamer heads is required, for example in the range of 25 to 50 meters. This configuration is difficult to achieve using conventional front-end gears. An exemplary application for this type of acquisition is the configuration composed of 20 streamers, each streamer being 6000 m long and having a separation between streamers of 25 m, i.e., 20×6000×25.
For very wide and low density acquisitions a very large separation is to be used for streamers, in the order of 200 to 300 meters. Here too there are difficulties when using the conventional front-end gears. A typical application in this case would be one with 10 streamers, each streamer being 10000 m long and the separation between the streamers is in the range of 300 m, i.e., 10×10000×300.